Semi-skilled high tensile strength steels

ABSTRACT

SEMI-KILLED STEEL COMPOSITION EXHIBITING HIGH TENSILE STRENGTH AND CONTAINING MANGANESE AND AT LEAST ONE OF THE METALS COLUMBIUM AND VANADIUM WHILE BEING DEVOID OF LARGE NON-METALLIC INCLUSIONS. THE SEMI-KILLED STEEL COMPOSITION IS CHARACTERIZED IN THAT IT HAS BEEN DEOXIDIZED BY ALUMINUM ALONE WHEREBY THE FORMATION OF INCLUSIONS OF MANGGANESE SILICATE LARGER THAN 100U IS AVOIDED. FURTHER, THE ALUMINA INCLUSIONS FORMED BY THE ALUMINUM DEOXIDATION ARE NOT LARGER THAN 100U. THERE IS LITTLE TENDENCY TOWARD CRACK FORMATION IN ARTICLES MANUFACTURED FROM THE INSTANT STEEL COMPOSITION.

April 3, 1973 SUSUMU SATOH ETAL 3,725,049

SEMI-KILLED HIGH TENSILE STRENGTH STEELS Filed Aug. 12, 1970 3 Sheets-Sheet 1 HOOK CRACK SEEN AT THE WELDED MRTION DURING PIP! PRODUCTION "FIG. I

CROSS SECTION OF STEEL PIPE ATTACKED BY HOOK CRACK INVENTORS 0 HIROSHI HORIGUCHI A TI'ORNEYS Apr 3, 1973 susuMu SATOH L 3,725,04

SEMI-KILLED HIGH TENSILE STRENGTH 'STEELS INCLUSIONS nv cscolvmuvwa sen/mafia STEEL max/0125a Br .s/ucou L esa/an, x100) FIG 5(1) mans/ans in zuvemve-sresL FIG. '5 7 (H) VEN TORS y SNUJIRO ONO M/EOWHI #mwuam A tram 5' United States Patent SEMI-SKILLED HIGH TENSILE STRENGTH STEELS Susumu Satoh, Muroran, Hideki Tanaka, Tokyo, and

Shujiro Ono and Hiroshi Horiguchi, Muroran, Japan,

assignors to Nippon Steel Corporation, Tokyo, Japan Continuation-impart of abandoned application Ser. No.

617,112, Feb. 20, 1967. This application Aug. 12, 1970,

Ser. No. 63,307

Claims priority, application Japan, Mar. 11, 1966, 41/15,060; July 21, 1966, 41/47,892 Int. Cl. C22c 39/26, 39/54 US. Cl. 75-123 D 5 Claims ABSTRACT OF THE DISCLOSURE CROSS-REFERENCE TO PRIOR APPLICATIONS This is a continuation-in-part of our copending application, Ser. No. 617,112, filed on Feb. 20, 1967, now abandoned.

SUMMARY OF INVENTION This invention generally relates to steels and is particularly directed to novel non-tempered semi-killed steel compositions exhibiting high tensile strength while being devoid of large non-metallic inclusions and containing manganese and at least one of the metals columbium and vanadium. The invention is also concerned with a process for producing the new steel compositions, the inventive steels being particularly suitable for use in the manufacture of steel plates and pipes. An essential characteristic of the novel semi-killed steels is that they have been deoxidized by aluminum alone.

It has previously been suggested to produce non-tempered, high tensile strength steel by adding suitable amounts of columbium and/or vanadium to semi-killed steel. The semi-killed steel to which columbium and/or vanadium are added in accordance with the prior art procedures is, without exception, deoxidized by silicon or by silicon and aluminum. No prior art columbium and/or vanadium containing high tensile strength semi-killed steel is known in which the deoxidation is carried out in the absence of silicon and by aluminum alone.

Conventional semi-killed high tensile strength steel plates containing columbium and/or vanadium are used for various purposes as, for example, in the manufacture of pipe lines for conveying oil and the like fluids, in the manufacture of motor car frames and the like.

The production of pipes from such conventional steel is, however, fraught with difliculties due to crack defects which tend to occur in the pipes. These crack defects, generally referred to in the art as hook cracks, are frequently formed in the welded portion, a fact which of course, lowers the yield.

In respect to frames and other structural members formed from such conventional steel plates, again cracks frequently occur at the corners upon bending of the plates by 90.

Extensive investigations have been carried out and tests have been performed by the present inventors with a 3,725,049 Patented Apr. 3, 1973 view to ascertaining and pinpointing the causes for these crack defects which occur upon working of steel plates made from the conventional prior art steels of the indicated kind. These investigations and tests have established the following facts: If a semi-killed steel is produced by deoxidizing the heat in the furnace, ladle or mold with silicon or silicon and aluminum, large silicate inclusions are formed which are not uniformly distributed throughout the heat and thus the solidified ingot, but which have a tendency to lodge themselves in the skin portion of the ingot. Consequently, when the ingot is hot rolled, these relatively large inclusions are stretched or elongated in the rolling direction and appear in the final steel product in the form of manganese silicate inclusions near the surface of the product. It has been established that the hook cracks previously referred to and occurring during pipe line production and the crack defects formed during bending in frame production, are due to the presence of these relatively large manganese silicate inclusions.

Steel pipes, particularly electrically welded steel pipes, are normally produced as follows: Generally, hot coil steel is formed into a pipe by a pipe mill and welded together, whereupon the welding seam or joint is subjected to various severe examinations and tests such as supersonic inspection for the purpose of detecting defects in the welded portions. These examinations also include hydrostatic testing, fiat testing, expansion testing and the like.

In the accompanying drawings:

FIG. 1 is a photographic rendering showing hook cracks which occurred at the welded portion during pipe production;

FIG. 2 is a photographic rendering showing a cross section of the welded portion in which the hook crack developed during pipe production, the hook crack being shown in x magnification;

FIG. 3(1), (II) and (III), respectively, shows the development of hook cracks;

FIG. 4 is a graph indicating the relationship between the bending quality and inclusions in carbon semi-killed steel plates deoxidized by aluminum and by silicon, respectively; and

FIG. 5 (I) is a photographic rendering showing inclusions in the rolling direction (L direction) in a conventional columbium-containing steel, while FIG. 5 (II) is a photograph showing corresponding inclusions in the L direction in a steel in accordance with this invention, both FIG. 5(1) and FIG. 5(II) being shown in 100x magnification.

A hook crack is a kind of hairline crack which appears in the bead of a pipe extending lengthwise of the pipe, as seen in FIG. 1. When this defective portion of a pipe is cut transverse to the lengthwise direction of the pipe and viewed under a microscope, it is noted that the defective portion contains relatively large inclusions of manganese silicate of more than 10011. in length, the crack ocurring along these manganese silicates as apparent from FIG. 2.

The following conclusions can be reached from the above observations in respect to the causes for the hook crack formation:

In respect to conventional semi-killed steels containing columbium and/or vanadium in which the steel has been deoxidized by silicon alone or by silicon and aluminum, the manganese silicate inclusions, during the plate rolling, are present near the surface of the plate in the manner shown in FIG. 3(1). Due to the presence of these manganese silicate inclusions of relatively large size, the steel mass immediately adjacent and enveloping the inclusions is subjected to larger stress forces during bending and pressure welding than the remaining steel mass. Therefore, the strata immediately below the surface of the plate are prone to crack formation, the cracks developing from the surface of the plate to the location of the inclusions whereupon the cracks proceed along the inclusions. A fresh crack oftentimes occurs from the metal mass adjacent the inclusion and extending toward and into the centrally located inner portion of the metal.

FIG. 3( II) illustrates the conditions in respect to a bead obtained by resistance welding in which the metal is shown to flow. As indicated in FIG. 3(III) the head is removed by the bead cutting operation after welding and a crack appears in the bead region when the head is cut away due to the presence of large inclusions, the crack being in the nature of a hook crack.

As previously set forth, if a columbium and/or vanadium-containing steel plate which has been deoxidized in the conventional manner by silicon or silicon and aluminum is pressure-bent into a motor car frame or the like, cracks have a tendency to occur in the corner portions of the pressed material. The mechanism causing the crack formation is very similar to that of the hook crack formation as explained above in the forming of plates into pipes. Large manganese silicate inclusions which are located adjacent the surface of the steel plate or sheet are subjected to considerable stress during the press working and become a stress-concentration source.

It is thus clear that the causes for the hook cracks in the welded portion of pipe lines and the cracks occurring during bend working of frames must be attributed to the large manganese silicate inclusions which are present in the conventional steel plates or sheets near the surface thereof.

These phenomena can also 'be observed during the working of an ordinary plain carbon semi-killed steel plate. However, the crack formation is more pronounced in a high tensile strength steel plate enriched with columbium and/ or vanadium, because the stresses to which the sheet is subjected during the working are much larger. The formation of the cracks referred to is well known in the industry and is a dreaded phenomenon which has not been solved and overcome prior to the invention in spite of serious efforts in this direction. The formation of these cracks renders a large proportion of manufactured pipes or frames defective which, of course, significantly lowers the production yield.

Accordingly, it is a primary object of the invention to overcome the drawbacks and disadvantages of prior art high tensile strength steels containing columbium and/or vanadium and to produce steel of the indicated kind which can be worked into pipes and bent into frames and the like structural members without risk of crack formation.

Another object of the invention is to provide a steel composition of the indicated kind which is devoid of alumina and manganese silicate inclusions larger than 100 4.

It is also an object of this invention to produce semikilled high tensile strength steel of the indicated nature which has superior characteristics in many respects as compared to prior art steels.

Still another object of the invention is to provide an improved process for manufacturing high tensile strength steel of the indicated kind.

It is also an object of this invention to produce novel columbium and/ or vanadium-containing high tensile strength steels which can be readily worked into pipes or structural members with a high production yield.

Generally, it is an object of this invention to improve on the art of high tensile strength semi-killed steels as presently practiced.

From the above it will be apparent that, with a view to eliminating crack formations of the indicated kind, it is necessary to reduce or prevent the formation of large manganese silicate and alumina inclusions within the steel mass.

Theoretically, this could possible be accomplished in the following two manners:

(1) The deoxidation degree could be intensified so as to prevent secondary deoxidation reactions within the mold.

(2) To reduce the size of the inclusions by varying or altering the nature of the deoxidizing agents without, however, increasing the deoxidation degree.

In respect to alternative 1 above, it should be noted that in respect to semi-killed steels of a customary degree of deoxidation, in which the deoxidation is accomplished by silicon alone or by silicon and aluminum, the oxygen concentration in the molten steel to be poured into a mold is not sufiiciently decreased so as to avoid a secondary deoxidation reaction with the silicon. In fact, such secondary reaction takes place as the temperature of the molten steel decreases and during the solidification, resulting in the formation of a considerable amount of manganese silicate which thus remains in the solidified ingot in the form of inclusions. Therefore, if silicon deoxidation is performed, it is extremely difficult, if not impossible, to prevent secondary deoxidation in the mold within the deoxidation range of semi-killed steel.

Consequently, if the deoxidation is carried out with silicon and large manganese silicate inclusions are to be avoided in the steel ingots, it is necessary to increase the silicon addition to the furnace or the ladle so as to completely kill the molten steel and prevent the occurrence and progress of secondary deoxidation reactions due to the presence of silicon. However, if this is done, considerable pipe formation takes place in the semi-killed steel in the top portion of the steel ingot which in turn negates the specific advantage of semi-killed steel, to wit, the attainment of higher yields. For this reason, it is not practically feasible to reduce the large inclusions near the surface of the steel plate while at the same time attaining a deoxidation degree with the deoxidation range for ordinary semi-killed steels. In practice, the avoidance of the large inclusions would therefore at the same time result in a killed steel.

Referring now to alternative 2, as mentioned above, the inclusions which are formed in the silicon deoxidation are large-sized manganese silicates, while by contrast in deoxidation carried out with aluminum alone, any inclusions which are formed consist of alumina (A1 0 and heroynite (FeO A1 0 Such alumina or heroynite inclusions are much finer and of smaller size than manganeses silicates. FIG. 5 (I) shows inclusions as they occur in a conventional semi-killed steel plate containing columbium and/or vanadium, the steel having been deoxidized by silicon or silicon and aluminum, while FIG. 5 (II) shows inclusions in an inventive semikilled steel plate containing columbium and/or vanadium, wherein the deoxidation has been carried out with aluminum alone. For the reasons advanced, and even if inclusions appear at a similar position near the surface, as do the manganese silicates in the conventional steels, these inclusions do not materially affect the stress conditions and no stress concentration takes place. Accordingly, since the inclusions are of much smaller size and do not exceed 100 as seen in IG. 4, no cracks are formed during the forming into pipes or bending of the plates into frame structures.

The fact that finer inclusions do not result in stress concentrations near the surface of the plate but that stresses are substantially uniformly distributed is illustrated in FIG. 4. The graph shows that 75% of the cracks occur when a steel plate of 3.2 mm. (0.125 inch) is bent during a laboratory bending test, if large manganese silicates are present at a depth within 0.2 mm. (0.008 inch) from the surface. By contrast, no cracks could be observed in the presence of the small alumina inclusions formed by the deoxidation with aluminum alone.

It has been established from the above observations that deoxidation carried out with aluminum without silicon yields significantly more advantageous results if proceeding according to alternative 2 above than if the deoxidation is carried out with silicon or with silicon and aluminum.

For the reasons advanced, alternative I referred to above is impractical, if a semi-killed steel is to be produced, while the deoxidation with aluminum alone in accordance with alternative 2 yields excellent results. For this reason, the present invention proceeds according to alternative 2, the deoxidation being carried out with aluminum alone.

However, if proceeding in this manner, to wit, if the deoxidation is carried out with aluminum, the control of the deoxidation reaction is somewhat more difficult than when deoxidation is carried out in the presence of silicon. Therefore, care must be taken in practicing the inventive process so as to make sure that during the deoxidation no large pipe formation occurs in the ingot.

Briefly, and in accordance with the invention, a novel non-tempered high tensile strength steel is therefore produced by adding columbium and/ or vanadium to a semi- -l illed manganese-containing steel which has been prepared by the deoxidation with aluminum alone. It should be emphasized that deoxidation by aluminum alone has not previously been attempted in connection with steels of the indicated nature.

Extensive tests have established that best results are achieved if the aluminum content in the final steel cornposition does not exceed 0.02%, calculated as acid soluble aluminum. The reason for this limitation is that a low content of aluminum is sufiicient in an aluminum deoxidized semi-killed steel as contrasted to an aluminum killed steel in which the aluminum is added for the purpose of regulating the grain size. Thus the upper limit for the aluminum content according to this invention is about 0.02%.

[In respect to the other alloying elements of the steel, the following should be observed:

The carbon content of the steel should be in the range of between about 0.06 to 0.30%, because a carbon content in an amount lower than 0.06% would be insufiicient to provide the required high tensile strength. On the other hand, carbon in excess of 0.30% lowers the elongation, drawability and impact value characteristics as well as the weldability. Higher carbon contents would, of course, increase the strength, but for the purposes of the present invention the other characteristics hereinabove referred to are more important. For this reason, it is desirable to maintain the carbon content as low as possible and the indicated range of 0.06 to 0.30 has proved to be most successful.

If the carbon content exceeds 0.30%, the ductility toughness deteriorates and the weldability is lowered and thus the upper limit of the carbon content should not exceed 0.30%.

Manganese is a useful element for increasing the strength as well as the impact value characteristics. However, amounts of less than 0.30% of manganese do not give sufficient strength in the same manner as to low a carbon content lowers the strength value. Manganese in an amount exceeding 1.5%, however, causes deterioration of the impact value and the weldability. Thus, the manganese range should be within 0.30 to 1.50%.

The columbium content should be 0.005 to 0.1% while the vanadium content is limited to 0.02 to 0.4% to obtain high tensile strength steel plates having 40 to 65 kg. per mm. (56.900 to 92.500 p.s.i.) tensile strength, and 25 to 52 kg./mm. (35.000 to 94.000 p.s.i.) yield strength as hot rolled A columbium content of less than 0.005% and a vanadium content lower than 0.02% would not be effective for increasing the strength of the product, while on the other hand columbium in excess of 0.1% and vanadium in excess of 0.04% does not result in any significantly increased strength.

.The optimum columbium and vanadium contents in the steel depend, of course, on the carbon and nitrogen contents in the steel. However, it has been ascertained that in respect to a low carbon steel containing a customary amount of nitrogen (0.002 to 0.01%), the best results in respect to strength of the product are obtained if the columbium content is within the range of 0.01 to 0.06% with the vanadium content being within the range of 0.05 to 0.30%. The columbium is advantageously added in the form of Fe-Cb alloy such as, for example,

plain Fe-Cb or Fe-Cb-Al. The columbium addition in this form is made to the ladle or the mold.

The vanadium is preferably added to the ladle or mold in the form of Fe-V or as Fe-V alloys with other elements, to wit, aluminum (Fe-V-Al).

As previously set forth, no silicon is used as deoxidizer since silicon is the main cause for the undesired manganese silicate inclusions. Therefore, any silicon which may be present in the steel is derived from unavoidable contaminations and impurities during the steel making. No positive silicon addition is effected. The upper limit of the silicon content in the steel should not exceed 0.02%.

Depending on the use to which the steel is to be put and on the requirements for the steel, other elements may optionally be added, such as, for example, copper phosphorus and chromium. Copper is a relatively cheap element effective for obtaining desired strength and for increasing atmospheric resistance. On the other hand, relatively large addition of copper tends to cause cracks during hot rolling, thus rendering production ditfcult. For this reason, it has been established that the copper content should not exceed 0.5%

In respect to phosphorus, this element, particularly in conjunction with copper, is etfective for increasing atmospheric resistance but, again, excessive amounts of phosphorus negatively affect workability at room temperature. It has been established that the phosphorus content should not exceed 0.10%.

Chromium is effective for increasing the tensile strength and when present in conjunction with copper is effective for increasing atmospheric resistance. The optimum content of chromium should not exceed 1.00%.

Pursuant to the invention, steel containing the above elements in specified amounts is rolled or formed into ingots or billets or the like shapes and then is hot rolled in customary manner. The steel is heated at a temperature of above 1160 C. (2120 F.) for two to six hours and hot rolled at a final finishing temperature of between about 750 to 900 C. (1380 to 1652 F.) and coiled at a temperature between about 350 to 700 C. (660 to 1292 F.) Heating to above 1160 C. causes columbium and vanadium to dissolve into austenite and to form precipitates which cohere to the matrix after the hot rolling, thus strengthening the steel. Therefore, the desired properties are obtained by combination of the above heating temperature, final finishing temperature and coiling temperature according to the requirements as to the strength or ductility rather than strength.

No large-sized manganese silicate inclusions are formed in the present steel since silicon is not employed as deoxidizer, deoxidation being carried out by the aluminum. The only inclusions which can be observed in the present steel are fine spherical A1 0 particles of a size less than 1001.0. Therefore, no hook cracks occur in the welded portions of steel pipes produced from the steel and no cracks or failure takes place during bending operations. Consequently, the present invention completely eliminates the difliculties previously met with conventional semikilled high tensile strength steels containing columbium and/or vanadium in which the deoxidation is carried out with silicon or silicon and aluminum. The present invention makes possible the production of a semi-killed high tensile strength steel plate containing columbium and/or vanadium with excellent workability.

The steel plate produced according to the present invention exhibits superior toughness, particularly in the right angle direction to its rolling direction, as will become apparent more clearly from the following Example 1. This is due to the fact that in the ordinary prior art semi-killed steel plates large manganese silicates are present in the rolling direction and thus orientation is pronounced while, by contrast, in the steel plate produced according to the present invention the inclusions of fine and non-oriented alumina A1 particles increase. the toughness.

The invention will now be described by several examples, it being understood, however, that these examples are given by way of illustration and not by way of limitation and that many changes may be eifected without affecting in any way the scope and spirit of the invention as recited in the appended claims.

EXAMPLE 1 The following ingot making conditions for steel (convertor steel) produced according to the present invention prevailed:

Ladle analysis in percent: C, 0.16; Si, 0.01; Mn, 0.47;

Amount of Al for ladle deoxidation: 320 g./t. (0.700

lb./t.)

Amount of Al for mold dexidation:

Mold No.3: 121 g./t. (0.270 lb./t.) Mold No.4: 115 g./t. (0.255 lb./t.) Mold No.5: 115 g./t. (0.255 lb./t.) Mold No. 6: 110 g./t. (0.240 lb./t.) Tapping temperature: 1610 C. (2930 F.) Ingot weight: 14,400 kg.

After the molten steel was poured from the ladle into the molds, Fe-Cb (Cb 61.0%) was added to the heat of Molds Nos. 4, and 6. The addition of Fe-Cb was started 10 seconds after the beginning of pouring and continued uniformly up to the completion of the pouring. Fe-Cb was crushed into particle size of less than 10 mm. (0.39 inch) and added in amounts tabulated below. Fe- Cb was added into Molds Nos. 4, 5 and 6 in an amount of kg./t. (0.730 lb./t.), 1 kg./t. (2.20 lb./t.) and 1.5 kg./t. (3.30 lb./t.), respectively.

Mold No. 3 contained comparison steel to which no Fe-Cb was added.

The steel ingots thus obtained were subjected to breaking down rolling into slabs which latter were heated at 1260 C. (2300 F.) and rolled into plates of a thickness of 6.0 mm. (0.236 inch) by a continuous hot rolling mill under the same conditions: Final finishing temperature: 840 C. (1550 F.) and coiling temperature: 600 C. (1110 F.). The chemical compositions and mechanical properties of the steel plates thus produced are shown in Table 1. The tension test results as shown in Table 1 (as well as in Tables 2, 4, 5, 14 and 17) were obtained in the rolling direction, and test pieces were taken in the rolling direction. The standard distance of elongation was 50 mm. (2 inches) (the test pieces for the results tabulated in Tables 2, 4, 5, l1, 14, 17 and 20 were made in the same manner). The specimens for impact tests were taken at right angle to the rolling direction and had a size of 5 x 10 x 55 mm. (0.197 x 0.394 X 2.17 inches). They were tested with 2 mm. (0.079 inch) V notch at 0 C. (32 F). (The same procedures were followed in the tests tabulated in Tables 2, 4, 5, 11, 14, 17 and 20.)

TENSION TEST RESULTS Impact Fe-Cb 'I.S., Y/P., Elongavalue, addition, kgJmrn. kgJmmJ tion, kg./m. Mold N0. kg./t. (p.s.l.) (p.s.i.) percent (it.-lb.)

By contrast, ingot making conditions for steel (convertor steel) according to the conventional procedure are as follows:

Ladle analysis in percent: C, 0.15; Si, 0.05; Mn, 0.48; P,

Ladle deoxidation: Fe-Si 500 g./t. (1.10 lb./t.)

Furnace deoxidation: Si-Mn 3900 g./t. (8.60 lb./t.)

Tapping temperature: 1600 C. (2912 F.)

Ingot weight: 14,400 kg.

During the pouring of molten steel from the ladle into the molds, /3 kg./t. (0.730 lb./t.), 1 kg./t. (2.20 lb./t.), and 1.5 kg./t. (3.30 lb./t.) of Fe-Cb was added to Molds Nos. 4, 5 and 6, respectively. The addition time and the particle size of Fe-Cb and the conditions of the subsequent hot rolling operation were the same as in the case of the inventive steel. No Fe-Cb was added to Mold No. 3, which was used as a comparative or control steel.

The chemical composition and mechanical properties are shown in Table 2.

TABLE 2 Chemical composition (percent) 0 Si Mn Cb Sol. Al

0. 15 0. 06 0. 50 None 0. 16 0.06 0. 51 0. 010 None 0. 15 0.07 0. 49 0.035 None 0. 15 0.07 0. 50 0.049 None TENSION TEST RESULTS Impact Fe-Cb T.S., Y/P Elongavalue, addition, kgJmmJ kgJmrn. tion, kgJm. Mold No (kg./t.) (p.s.i.) (p.s.i.) percent (IL-lb.)

As apparent fromthe results shown in Table 1 and Table 2, there is no substantial difference in strength and elongation between the inventive steel, prepared by adding columbium to semi-killed steel deoxidized solely by aluminum, and the conventional steel prepared by adding columbium to silicon deoxidized semi-killed steel. However, there is a distinct significant difference in impact value, and the steels produced according to the present invention have excellent toughness in the direction at right angle to the rolling direction. The inventive steel and the conventional steel were examined under a microscope and measured as the number of inclusions of more than 100,11. length in 60 fields. The results are shown in Table 3.

TABLE 3 TABLE 1 Chemical composition (percent) 7 Number of inclusions of more th 0 Si Mn Cb Sol. Al 1n 6 0 fi i d s length Mold 0.17 0.01 0.48 0.011 Type of steel N 0.10 0.01 0.48 0.009 0.010 0 Top Middle Bottom 0. 17 0. 01 0. 47 0. 032 0. 009 Inventive steel 5 0 0 0 0. 17 0. 01 0. 48 0. 047 0. 006 Conventional steel 6 19 14 15 As seen from the results in Table 3, in the inventive steel, large manganese silicate inclusions which cause hook cracks and bending cracks were reduced almost to nil.

From the steels of Table 3, bending test specimens of 100 mm. width and 200 mm. length (3.94 inches x 7.88 inches) were taken in the right angle direction to the rolling direction, and their critical bend radius was determined with various bending radii. The inventive steel could be bent 180 with a bend radius of 1.0 x plate thickness, whereas the conventional steel could be bent 180 only with a bend radius of 1.5 x plate thickness. FIG. 5 is a photograph showing the inclusions in these two specimens. The inclusions shown in FIG. 5 (I) are of manganese silicates present in the conventional steel plate (Mold No. 5) which was produced by adding columbium to silicon deoxidized semi-killed steel. The inclusions shown in FIG. 5(II) are of the alumina of a size less than 10011. in the inventive steel (Mold-No. 5) which was produced by adding columbium to semi-killed steel deoxidized by aluminum alone. The difference between these two types of inclusions causes the difierence in the impact value for the steels in the right angle direction to the rolling direction and the bending quality.

EXAMPLE 2 A heat having the following ladle analysis was prepared (percent): C, 0.27; Mn, 1.00; Si, 0.01; P, 0.01; S, 0.016.

Ingots were made from this composition under the following conditions:

Amount of Al for ladle deoxidation: 205 g./ t. (0.452

lb./t.) Amount of Al for mold deoxidation:

Mold No. 3 100 g./ t. (0.220 lb./t.) Mold No. 4 97 g./t. (0.02151b./t.) Mold No. 5 97 g./t. (0.215 lb./t.) Tapping temperature: 1605 C. (2920 F.) Ingot weight: 14,400 kg.

Fe-Cb was added to Mold Nos. 4 and 5 in an amount 01% kg./t. (1.10 lb./t.) and 1 kg./t. (2.20 lb./t.), respectively. These ingots were hot rolled into plates of 6.0 mm. thickness (0.236 inch) under similar conditions as in Example 1. The chemical composition and mechanical properties of these plates are shown in Table 4.

TABLE 4 Fe-Cb Chemical composition (percent) Mold addition, No. {kg.lt.) C Si Mn Cb Sol. Al

TENSION TEST RESULTS Impact Fe-Cb T.S., Y/P Elongevalue, addition, kgJmm. kgJmm 1 tion, kgJm. Mold No. (kg./t.) (p.s.i.) (p.s.i.) percent (in-lb.)

The ingot making conditions for steel (convertor steel) according to the conventional method are as follows: Ladle analysis (percent): C, 0.26; Si, 0.06; Mn, 0.98;

Ladle deoxidation: Fe-Si: 700 g./t. (1.54 lb./t.) Tapping temperature: 1610 C. (2930 F.) Ingot weight: 14,400 kg.

During the pouring into molds, Fe-Cb was added to Molds Nos. 4 and 5 in an amount of V2 kg./t. (1.10 lb./t.) and 1 kg./t. (2.20 lb./t.), respectively. These ingots were hot rolled to a thickness of 6.0 mm. (0.236

10 inch) under similar conditions as in the case of the inventive steel. Chemical analysis and mechanical properties of these plates are tabulated in Table 5.

TABLE 5 Fe-Cb Chemical composition ercent Mold addition, (p No. (kg./t.) C Si Mn Cb Sol. Al

3 0 0. 27 0. O8 0. 97 None 4 16 0. 27 0. 07 0. 98 0. 010 None 5 1 0. 28 0. 07 0. 98 0. 031 None TENSION TEST RESULTS Impact Fe-Cb T.S., Y/P., Elongavalue, addition, kg./mm. kgJmmJ tion, kg./m. Mold No (kg./t.) (9.5.1.) (p.s.i.) percent (it.-1b.)

As seen from the results of Tables 4 and 5, the inventive steel is excellent in respect to toughness in the right angle direction of the rolling direction. The steel produced according to the present invention and the steel produced according to the conventional method were inspected under a microscope to determine the number of inclusions of a length of more than 100 u. The results are shown in Table 6.

TABLE 6 Number of inclusions of more than 1001.1 length in 60 fields Mold Type of steel No. Top Middle Bottom Inventive steel 4 0 0 0 Conventional steel 4 16 10 8 As in the steel of Example 1, the inventive steel was devoid of largesize manganese silicate inclusions.

EXAMPLE 3 A high tensile strength steel plate containing columbian and produced according to the present invention and similar steel produced according to the conventional method were delivered to a steel pipe manufacturer. The ingot making conditions and rolling conditions for the steels are shown below. Fe-Cb was added to the ladle.

TABLE 7 Ladle analysis (percent) Type of steel C Si Mn P S Cb Inventive steel 0. 22 0.01 0.94 0.021 0. 023 0. 026 Conventional steel- 0.21 0.06 0. 94 0. 015 0.013 0. 024

TABLE 8 Addition of deoxidation agent Furnace Ladle Mold Type of steel addition addition addition Inventive steel A1, 300 g./t. A1, 61 g./t.

(0.660 lb./t.). (0.134 lb./t.). Conventional steel Fe-Si, 1,000 g./t.

(2.20 lb./t.).

TABLE 9 Hot rolling temperature, C. (*F).

Final Heating finishing Coiling Type of steel temperature temperature temperature Inventive steel 1. 280 850-865 600 (2, 340) (1, 5601590) (1, Conventional steel 1, 280 845-865 610 (2,340) (1,550-1, 590) (1,130)

Both steels were rolled into coil form of 6.0 mm. (0.236 inch) thickness and 1214 mm. (4 feet) width. Table 10 shows the chemical composition of these plates. (Mold No. 3 was regarded as representative.)

. 12 the results of microscopic measurement of the steel inclusions. No. 1 and No. 2 in each of the tables indicate conventional columbium-containing steels and N0. 3

TABLE 10 to No. 8 refer to the inventive steel plates. No. 3 and Pop Chenflcalmmmsmm (Percent) No. 4 are columbium-containing steels while No. 5 and Type ofsteel tlon 0 s1 Mn P s Cb 361.41 No. 6 are vanadium-containing steels. No. 7 and No. 8 Inventive T 0.19 0.007 0.97 0.013 0.014 0.023 0.007 are steels contammg both columbum' and vanadlllm' steel. M 0.21 0.009 0.97 0.015 0.015 0.022 0.008 These steel plates were produced from different B M14 M14 charges, but under production conditions as similar as Convrintional g 0.32 3. 3% gone 10 possible. The hot rolling and mechanical testing condistee 0. one

B 0.19 0.080 1.02 0.009 0.014 0.020 None Hons were Sumlar to those of Example It should be noted that these steel plates have higher 111W? phosphorus, chrome and copper contents to increase their Bottom portion of mgot.

atmospheric resistance.

TABLE 13 Chemical composition (percent) 0 Si Mn P 8 Cu Cr Cb v s61. .41

0.12 0.06 0.33 0.090 0.021 0.39 0.46 None 0. 11 0. 07 0. 34 0. 089 0. 019 0. 39 0. 46 None 0. 12 0. 01 0. 32 0. 087 0. 019 0. 38 0. 46 0. 006 0. 12 0. 01 0. 0. 089 0. 022 0. 38 0. 46 0. 007 0. 11 0. 01 0.33 0. 093 0. 021 0. 42 0. 42 0. 006 0. 12 0. 01 0. 34 0. 092 0. 024 0. 43 0. 43 0. 005 0. 12 0. 01 0. 32 0. 088 0. 019 0. 0. 45 0. 006 0. 12 0. 01 0. 35 0. 090 0. 020 0. 41 0. 47 0. 005

Table 11 indicates the mechanical properties of the steel plates of Table 10.

TABLE 11 Tensile strength, Yield point, Impact value, kgJmm. (p.s.i.) kg./mm. (p.s.i.) Elongation (percent) kg./m. (it./ib.)

Type of steel Portion L o L o L' c L c Inventive steel T 58.5 58.0 43.8 43.9 29.7 23.1' 5.2 2.8 (83, 200 (82, 500) (62,300) 62, 400 37. 5 20. 2) M 58.8 59.0 44.5 45.1 29.5 23.7 5.6 2.9 (83, 600) (84, 000) 63,400 64, 100 (40. 4 20. 9) B 57.2 57.8 43.5 44.2 29.9 27.4 5.0 2.7 (81, 400) 82, 200) (61, 900) (62, 800 I 36. 1) 19. 5

Conventional steel T 59.9 60.4 44.8 45.0 27.8 23.0 5.0 1.6 (85, 200) (95, 900) (63, 700) (64, 000) 36. 1 11. 5 M 60.1 61.3 45.9 47.8. 27.8 22.7 6.0 1.8 (85, 500) (87, 200) (65, 300 68, 000) (43. 3) 1a. 0) B 58. 45. 45.0 28. 1 25. 6 4. 9 1. 5 82,900 84,000 (64, 100) (64, 000 35. 4 10. 8)

NorE.--L=Rolling direction; C= Right angle to rolling direction.

50 tons of each of these two types of steels were TABLE 14 supplied to a steel pipe producer and processed into APl pipe (X-52) of 16 inch diameter. The results are Tension Impact shown in Table 12. T.S., Y.P., Elongavalue, 1 kgJmm. kg./mm. tion, kg./m. TABLE 12 50 Type of steel No. (p.s.l.) (p.s.i.) percent (it-lb.)

Conven- Conventional 1 48. 7 39.0 33. 9 2. 3 Inventive tional steel. (69, 300) (55, 400) (16. 6) steel steel 2 p 55.5 V n 64 28.7 1.7

78,900 700 11.6 Yield (percent) 88.69 41.10 Scrap (percent) 11. 31 58. 9 Inventive 3 48. 6 38. 2 35. 1 3. 5 Breakdown (percent): 55 steel; (69,100) (54,300) (25.3) Hook crack 1- 00 5 00 4 56.0 45. 0 28. 3 3. 1 Trimming 1088.--- 5. 00 5- 0 (79, 600) (64,000) (22. 4) Open seam 2- 0 30 5 47. 2 36. 8 36. 2 3. 7 Sampling loss-.. 1. 00 1. 00 (67, 100) (52, 300) (26. 8) Others 2. 31 1. e 55. 1 44. 1 30. 0 3. 2 "839 332 The above results demonstrate a significant difference 60 (70,160) (54,350) 5 in the yield percentage between the two types of steel. 8 3 3 3 This is due to the fact that the hook crack occurrence is only 1% in the steel produced according to the present invention while it is as high as 50% in the con- TABLE 15 ventional steel of which 25 tons are defective. The hook 65 Number of inclusions (more than cracks were sensed and detected by a supersonic detector 100,. length pos1t1oned on the producnon l1ne. Type Steel No. Top Middle Bottom EXAMPLE Conventional steel 1 16 10 8 Mechanical tests were conducted on an inventive semi- 2 17 11 12 killed steel plate containing colum'bium and/or vanadium Inventive steel 3 0 0 0 and a conventional semi-killed steel plate containing 3 3 3 3 columbium and/or vanadium. Table 13 shows the chemi- 6 0 0 0 cal composition of these steel plates, while Table 14 g 3 g 3 indicates their mechanical properties. Table 15 tabulates As clearly seen from the results in Table 14, there is no substantial difference in strength and elongation between the inventive steels, obtained by adding vanadium and/or columbium to semi-killed steel deoxidized by aluminum alone, and the conventional steels obtained by adding columbium to semi-killed steel deoxidized by silicon. However, there is a distinctive dilference in the results of the impact tests. The steels produced according to the presentinvent-ion show excellent toughness, particularly in the C direction.

Table 15 also demonstrates that in the inventive steels, large manganese-silicate above 100p inclusions are reduced to zero.

EXAMPLE Mechanical tests were conducted on conventional semikilled steels containing'vanadium, and semi-killed steels containing both vanadium and columbium according to the present invention.

Table 16 shows the chemical composition of the steel samples while Table 17 indicates their mechanical properties. Table 18 tabulates the results of measurements of inclusions larger than 100 1..

No. l and No. 2 are conventional vanadium-containing steels, No. 3 through No. 6 refer to inventive steels. No. 3 and No. 4 are vanadium containing steels. No. 5 and No. 6 are steels containing both vanadium and columbium.

These steels were produced and tested under the same conditions.

TABLE 18 Number of inclusions of more than 100;

length in 60 fields Type of steel No. Top Middle Bottom Conventional steel 1 16 8 9 2 20 9 4 Inventive steel 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0 TAB LE 16 Chemical composition (percent) Type of steel No. C Si Mn P S Ob V Sol. Al

Conventional steel 1 0. 27 0. 08 0. 46 0. 016 0. 030 None 2 0. 26 0. 09 0. 46 0. 015 0. 152 None Inventive steel 3 0. 26 0. O1 0. 49 0. 015 0.033 0.005 4 0. 27 0. 01 0. 4s 0. 017 0. 160 0. 006 5 0. 28 0. 01 0. 49 0. 014 0. 022 0. 006 6 0. 26 0. 01 0. 49 0. 015 0. 065 0. 005

TABLE 17 60 [Tensile strength testing results] Ex MPLE 6 pact Semi-killed high tensile strength steel plates produced g; Zfi-a fi fif according to the present method and semi-killed high p of Steel (p- (r percent (ftb) 65 tensile strength steel plates produced according to the conoonvenflonal ventional method were shipped to a steel pipe manufacsteel 1 -6 turer for further processing.

2 222 29 3M 1 Table 19 shows the chemical composition, Table 20 (6 indicates the mechanical properties, and Table 21 tabu- Invemve 70 lates the pipe production yields of the steel samples. In steel 3 48.9 38.4 35.5 3.6 these tables, No. l is a conventional vanadium-containing (69,500) (54,600) (26.1) 4 517 2&9 steel, No. 2 1s a vanadium-containing steel of the present invention, and No. 3 is an inventive steel containing co- 5 51. 5 40. 6 33.6 3.3 1 d "3,20% 57 7 29 4 uI'lilhlllm anl vana 111111.1 d d d 6 ese stee s were pro uce un er pro ucing conditions (82200) 100) (19'5) 75 as similar as possible.

TABLE 19 Chemical composition (percent) Sol. Type of steel No. Portion C Si Mn P V A] Conventional steel 1 Top 0. 20 0.07 0. 63 0. 022 0. 055 None Middle 21 08 63 0. 022 057 None Bottom 19 .07 .018 056 None Inventive steel 2 Top 0 01 021 060 008 Middle. 20 01 63 019 050 08 Bottom .19 .01 62 .018 059 .06 3 Top .20 .01 .62 .013 .064 .01 Middle .21 01 .60 012 062 08 Bottom 18 01 60 .012 062 06 TABLE 20 [Results of tension test] Elongation, Impact value, kg./m. 'I.S.,kg./n1m. (p.s.i.) Y.P.,kg./mm. (p.s.i.) percent (tn-lb.)

Type of steel No. Portion L C L C L C L Conventional steel 1 Top 53.2 53.4 44.0 44.1 32.0 28.6 5.5 1.8

(75, 500) (75, 900 (52,500) (52, 700) (39. s) (13. 0) Middle 54. 55.0 44. 2 44 s as. 2 29.0 5. 4 1. 5

(77, 500 75, 200) (52, s00) (53, 500) 39. 0 10. 8 Bottom 54 0 54. 5 45. 1 45.0 as. 1 29. 0 5. 4 1. 7 (75, 700) (77, 500) (54, 700) (54, 000) (39. o) (12. 3) Inventive steal 2 Top 54.6 54.6 43.3 44.0 32.6 27.8 5.8 3.2 (77, 500) (77. 500) (51, 500) (52, 500) (41. 9 (23. 1) Middle 55.0 55.7 44.5 45. 2 31. 4 27. 1 5. 5 a. 0

(78. 200) (79, 100) (55, 400) (54, 200) (40. 5) (21. 7) Bottom 54. 9 55. 0 44. s 45. 0 s2. 0 27. 9 5. 7 2. 9

(78, 000) (78, 200) (53, 500) (55, 400) (41. 2 (21. 0 a Top 54.2 54.7 44.8 45.4 33.4 29.7 5.8 3.5

(77,000) (77,800) (53, 500) (54,500) (41. 9) (25. a) Middle 54. s 55.5 45. 5 45. s 33. 2 27.9 5. 9 3. 4

(77. 900) (78, 990) (54, 500 (55, 100) (42. 7) (24. 5) Bottom 54. s 55. 3 45. 5 45. 9 as. 5 28. a 5. 5 a. 4

TABLE 21 copper, phosphorus and chromium in the following amounts: [Results of pipe production] C Cu: not more than 0.5%;

OIIVBII- tional Inventive Inventive 25 P not more than 010% tee steel steel 3) Cr: not more than 1.0%. Yield (percent) 4M3 88,2, 3 A steel plate made from the steel composmon of Scrap (percent) 54.77 14.58 11.80 013.1111 1 and substantially devoid of manganese s1l1eate Bregiliggy 21451:??? 42.50 1.50 1.20 inclusions near its Surfacerimming loss 20 M0 4. A steel plate as claimed in claim 3, additionally congg gg gfig g: I {88 {-33 i sisting of at least one of the elements copper, phosphorus Others 2.07 2.88 2.30 and chromium in the following amounts:

As clearly shown in Table 20, the mechanical proper- 35 ties in C direction of both of the present steels are better than those of the conventional steel.

There is a substantial dilference in the results of pipe production between the present steels and the conventional steel: in the conventional vanadium-containing steel the hook crack occurrence is 42.5%, whereas in the inventive steels it is reduced to 1.6% and 1.2%, respectively.

These hook cracks were detected by a supersonic wave detector placed on the production line.

It will thus be clearly understood that the inventive semi-killed high tensile strength steel plates have excellent properties.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

What is claimed is:

1. In a semi-killed high tensile strength steel consisting essentially of between about 0.06 to 0.30% of carbon, between about 0.30 to 1.5% of manganese, not more than 0.02% of silicon, not more than 0.02% of acid soluble aluminum, and at least one of the elements columbium and vanadium in the ranges Cb: about between 0.005 to 0.1% and V: about between 0.02 to 0.40% with the balance being essentially iron, the improvement which comprises that the steel is deoxidized by aluminum alone, that the alumina inclusions formed by the aluminum deoxidation are not larger than 10014 and that the steel is devoid of manganese silicate inclusions larger than 10011..

2. The improvement as claimed in claim 1, wherein the steel consist additionally of at least one of the elements Cu: not more than 0.5%;

P: not more than 0.10%; and

Cr: not more than 1.0%.

5. In a method of producing high tensile strength semikilled steel consisting essentially of between about 0.060.30% of carbon,

between about 0.301.5% of manganese,

not more than 0.02% of silicon,

not more than 0.02% of acid soluble aluminum, and at least one of the elements columbium and vanadium in the ranges Cb: about between 0.005-0.1% and V: about between '0.02-0.4%, with the balance being essentially iron, the improvement which comprises deoxidizing said steel to its semi-killed state solely with aluminum, whereby the formation of manganese silicate inclusions larger than 100 1 is essentially avoided and the alumina inclusions formed by the aluminum deoxidation are not larger than about 100,74.

References Cited UNITED STATES PATENTS 3,496,032 2/1970 Shimizu et a1. 148-12 3,010,822 11/1961 Altenburger -123 3,102,831 9/1963 Tisdale 75--l23 X 3,459,540 8/1969 Tisdale 75129 OTHER REFERENCES Metals Handbook Committee, Metals Handbook, 1948 edition, ed. by Taylor Litman, Cleveland, Ohio, ASM, pp. 12 and 324.

RICHARD O. DEAN, Primary Examiner J. E. LEGRU, Assistant Examiner US. Cl. X.R.

75-123 J, N, 124, 125, 1126 E, F, K, 129; 14836 

